CVJ is a member for evenly transmitting the power of an engine to rotate right and left wheels at a given velocity. With the recent tendency to a front engine front drive (FF) system of automobiles, CVJ has shown marked development.
When a tripod type joint or a double offset type joint slides while transmitting torque, slide resistance develops in the axial direction. If this resistance is great, vibrations from the engine or the road are transmitted to the automobile body, becoming the source of vibrations of the body and booming noise. While mechanical improvements against this problem have been added to CVJ itself, the frictional resistance of CVJ can be reduced by using low-friction grease, which is effective in lessening the vibrations and noise of automobiles.
Therefore, grease to be applied to CVJ, particularly plunging type CVJ is keenly required to reduce frictional resistance of the sliding part. Grease having a low coefficient of friction is capable of reducing the friction of CVJ and thereby preventing generation of vibrations.
In order to meet the above demand, cases are increasing in the market, in which urea grease having high heat resistance and excellent frictional wear characteristics is used. The grease compositions disclosed in JP-A-6-57283 (The term "JP-A" as used herein means an "unexamined published Japanese patent application") and JP-B-5-79280 (The term "JP-B" as used herein means an "examined Japanese patent publication") can be mentioned as typical examples.
The grease composition for CVJ according to JP-A-6-57283 is urea grease containing (a) molybdenum disulfide, (b) molybdenum dialkyldithiocarbamate sulfide, and (c) a lead dialkyldithiocarbamate.
The grease composition for CVJ according to JP-B-5-79280 comprises urea grease and, as additives, a combination of molybdenum dithiocarbamate and molybdenum dithiophosphate, or a mixture of these organomolybdenum compounds and zinc dithiophosphate.
These grease compositions available from those references, especially when applied to plunging type constant velocity joints, make the induced thrust force smaller than with commercially available grease. However, vibrations occur in the shaft, and they are not regarded as satisfactory low-frictional grease.
An object of the present invention is to provide a grease composition for CVJ which has a low coefficient of friction to decrease the vibrations of CVJ.